Synchronizing multiple over the top streaming clients

ABSTRACT

Systems and methods for synchronizing the playback of OTT or other time sensitive content on multiple playback devices is disclosed. The systems and methods include receiving time information based on a network time source in the playback devices. The playback clock in each playback device is set based upon the time information. Stream initiation information derived using the time information from the network time source is received by each of the playback device from the media provider. The playback devices use the stream initiation information to adjust the presentation time stamps of the frames of the media content in the stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/192,695, entitled “Synchronizing Multiple Over The Top StreamingClients”, filed Jun. 24, 2016, which application is a continuation ofU.S. application Ser. No. 13/906,952, entitled “Synchronizing MultipleOver the Top Streaming Clients”, filed May 31, 2013 and issued on Jun.28, 2016 as U.S. Pat. No. 9,380,099, the disclosures of which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention relates to the playback of streamed media content bymultiple playback devices. More particularly, this invention relates tosynchronizing the playback of media content by multiple playback devicesof Over The Top (OTT) or other presentation time sensitive mediastreamed over a network.

BACKGROUND OF THE INVENTION

The term streaming media describes the playback of media on a playbackdevice, where the media is stored on a server and continuously sent tothe playback device over a network during playback. For purposes of thisdiscussion, media and/or encoded media are defined as data of a workthat includes video, audio, pictures, or another type of presentationthat may be displayed, played or in some other way presented by aplayback device. Typically, the playback device stores a sufficientquantity of media in a buffer at any given time during playback toprevent disruption of playback due to the playback device completingplayback of all the buffered media prior to receipt of the next portionof media. Adaptive bit rate streaming or adaptive streaming involvesdetecting the present streaming conditions (e.g. the user's networkbandwidth and CPU capacity) in real time and adjusting the quality ofthe streamed media accordingly. Typically, the source media is encodedat multiple bit rates and the playback device or client switches betweenstreaming the different encodings depending on available resources.

Adaptive streaming solutions typically utilize either Hypertext TransferProtocol (HTTP), published by the Internet Engineering Task Force andthe World Wide Web Consortium as RFC 2616, or Real Time StreamingProtocol (RTSP), published by the Internet Engineering Task Force as RFC2326, to stream media between a server and a playback device. HTTP is astateless protocol that enables a playback device to request a byterange within a file. HTTP is described as stateless, because the serveris not required to record information concerning the state of theplayback device requesting information or the byte ranges requested bythe playback device in order to respond to requests received from theplayback device. RTSP is a network control protocol used to controlstreaming media servers. Playback devices issue control commands, suchas “play” and “pause”, to the server streaming the media to control theplayback of media files. When RTSP is utilized, the media server recordsthe state of each client device and determines the media to stream basedupon the instructions received from the client devices and the client'sstate.

In adaptive streaming systems, the source media is typically stored on amedia server as a top level index file pointing to a number of alternatestreams that contain the actual video and audio data. Each stream istypically stored in one or more container files. Different adaptivestreaming solutions typically utilize different index and mediacontainers. The Synchronized Multimedia Integration Language (SMIL)developed by the World Wide Web Consortium is utilized to create indexesin several adaptive streaming solutions including IIS Smooth Streamingdeveloped by Microsoft Corporation of Redmond, Wash., and Flash DynamicStreaming developed by Adobe Systems Incorporated of San Jose, Calif.HTTP Adaptive Bitrate Streaming developed by Apple Computer Incorporatedof Cupertino, Calif. implements index files using an extended M3Uplaylist file (.M3U8), which is a text file containing a list of URIsthat typically identify a media container file. The most commonly usedmedia container formats are the MP4 container format specified in MPEG-4Part 14 (i.e. ISO/IEC 14496-14) and the MPEG transport stream (TS)container specified in MPEG-2 Part 1 (i.e. ISO/IEC Standard 13818-1).The MP4 container format is utilized in IIS Smooth Streaming and FlashDynamic Streaming. The TS container is used in HTTP Adaptive BitrateStreaming.

Adaptive bitrate streaming performs adequately for playback of mediacontent on a device. However, there is currently no viable way tosynchronize the playback of streamed content on two or more playbackdevices such that there is no discernible difference between thepresentation times of the content on the two or more devices. Currently,the only way to synchronize the device is for the user to manuallycontrol the start and stop of the playback on each of the devices tosynchronize the playback which is often impossible or at least verydifficult to achieve. Furthermore, the streaming of Over The Top (OTT)content or other presentation time sensitive content using adaptive bitrate streaming and other streaming processes is becoming more prevalent.OTT content is content that a media content provider delivers but maynot have any control of the content and can only provide the content asit is received. The best example of OTT content is a live broadcast.During a live broadcast, the content is streamed directly to theplayback devices to be played shortly after it is received in real time.Due to differences in the capabilities of playback devices and thenetwork connections of the devices, the timing of receipt and playbackof the streamed content on different devices may not be synchronized. Assuch, the manual synchronization is even more difficult than it is forsynchronizing the playback of stored content on playback devices.

The lack of synchronization may be a problem when one or more devicesare performing the playback within close proximity of one another as thelack of synchronization may be noticeable by viewers. For example, twoor more playback devices may be set up in a restaurant to view a livesporting event. If the playback is not synchronized, the user may noticethe broadcast delay between devices.

SUMMARY OF THE INVENTION

Systems and methods for synchronizing the playback of streamed OTTcontent or other time sensitive content in accordance with embodimentsof this invention are disclosed. In accordance with embodiments of thisinvention, a method for playback of encoded media performed by aplayback device be performed by the following actions. The playbackdevice transmits a request to a network time provider. In response tothe request, the playback device receives the time information from thenetwork time provider. The playback device sets a playback clock in thedevice using the received time information. In order to provide aplayback of a stream of content, the playback device receives streaminitiation information for the stream of encoded media from a streammedia provider. The stream initiation information includes a streamstart time. The playback device then receives the stream of encodedmedia. The stream includes frames of the encoded content. Thepresentation time information of each of the frames of the stream isadjusted in the playback device based upon the stream initiationinformation.

In accordance with some embodiments of this invention the network timeprovider is a Network Time Protocol Server and the transmitting of therequest and receiving of the time information are performed usingNetwork Time Protocol (NTP). Furthermore, the time information and/orthe start time is provided in Coordinated Universal Time in accordancewith some embodiments this invention.

In accordance with some other embodiments of this invention, the networktime provider is a local time server. In accordance with some of theseembodiments, the local time server performs the following process toobtain time information. The local time server transmits a request fortime information to a Network Time Protocol server. In response to therequest, the local time server, receives the time information from theNetwork Time Protocol server in the local time server. The local timeserver then may receive the request for time information from theplayback device. The local time server then generates the timeinformation for the playback device from the timing information receivedfrom the Network Time Protocol server and transmits the time informationto the playback device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network diagram of a media streaming system inaccordance with an embodiment of the invention.

FIG. 2 illustrates a block diagram of components of a playback devicethat performs processes for providing systems and methods in accordancewith an embodiment of this invention.

FIG. 3 illustrates a block diagram components of a server that performsprocesses for providing systems and methods in accordance with anembodiment of this invention.

FIG. 4 illustrates a timing diagram of information transmitted betweendevices over a network in accordance with some embodiments of thisinvention.

FIG. 5 illustrates a timing of information transmitted between devicesover a network in accordance with other embodiments of this invention.

FIG. 6 illustrates a flow diagram of a process performed by a playbackdevice in accordance with embodiments of this invention.

FIG. 7 illustrates a flow diagram of a process performed by a streamcontent provider in accordance with embodiments of this invention.

DETAILED DISCLOSURE OF THE INVENTION

Turning now to the drawings, systems and methods for synchronizingplayback of OTT content by multiple clients is provided. In particular,embodiments of this invention can use time information from a commonsource to set the playback clocks in the individual playback devices.The playback devices then receive start time information from a mediacontent provider that indicate the time for the start of playback. Thestart time information is determined using the time information from thecommon source. Each device then adjusts the presentation time stamps ofthe frames in the received content based on the received start timeinformation.

Streaming System Architecture

Turning now to the FIG. 1, an adaptive streaming system in accordancewith an embodiment of the invention is illustrated. The adaptivestreaming system 10 includes a source encoder 12 configured to encodesource media as a number of alternative streams. In the illustratedembodiment, the source encoder is a server. In other embodiments, thesource encoder can be any processing device including a processor andsufficient resources to perform the transcoding of source media(including but not limited to video, audio, and/or subtitles).Typically, the source encoding server 12 generates a top level index toa plurality of container files containing the streams, at least aplurality of which are alternative streams. Alternative streams arestreams that encode the same media content in different ways. In manyinstances, alternative streams encode media content (such as but notlimited to video) at different maximum bitrates. In a number ofembodiments, the alternative streams are encoded with differentresolutions and/or at different frame rates. The top level index fileand the container files are uploaded to an HTTP server 14. A variety ofplayback devices can then use HTTP or another appropriate statelessprotocol to request portions of the top level index file and thecontainer files via a network 16 such as the Internet.

Time server 22 is a server that provides Coordinated Universal Time(UTC) or equivalent information. In the shown embodiment, the timeserver 22 is a Network Time Protocol (NTP) server or an equivalentsystem that may be maintained by a third party. The use of NTP allowsthe time server 22 to provide time information to the devices allowinternal clocks in the individual devices that are set using the timeinformation to be synchronized within tens (10 s) of milliseconds (ms)of one another.

In the illustrated embodiment, playback devices include personalcomputers 18, CE players, and mobile phones 20. In other embodiments,playback devices can include consumer electronics devices such as DVDplayers, Blu-ray players, televisions, set top boxes, video gameconsoles, tablets, and other devices that are capable of connecting to aserver via HTTP and playing back encoded media. Although a specificarchitecture is shown in FIG. 1, any of a variety of architecturesincluding systems that perform conventional streaming and not adaptivebitrate streaming can be utilized that enable playback devices torequest portions of the top level index file and the container files inaccordance with embodiments of the invention.

As illustrated, some playback devices, such as user computers 29, areconnected to the network 16 via a Local Area Network (LAN) 25. LAN 25may include a local time server 27. Local time server 27 receives timeinformation from the NTP server 22 and generates local time informationbased upon the time information received from the NTP server 22.Although shown as a server, one skilled in the art will recognize thatany device connected to LAN 25 that can provide time information may beused without departing from embodiments of this invention. The localtime information of local time server 27 may be provided to the playbackdevices connected to the LAN 25 to ensure that better timesynchronization between the connected devices. The use of local timeserver 27 connected to the LAN 25 may be used when a synchronization ofless than the 10 s of ms achieved using network time server 22 isdesired. Network time servers, playback devices, and local time serversin accordance with embodiments of the invention are discussed furtherbelow.

Playback Device

Some processes for providing methods and systems in accordance withembodiments of this invention are executed by a playback device. Therelevant components in a playback device that can perform the processesin accordance with an embodiment of the invention are shown in FIG. 2.One skilled in the art will recognize that playback device may includeother components that are omitted for brevity without departing fromthis invention. The playback device 200 includes a processor 205, anon-volatile memory 210, and a volatile memory 215. The processor 205 isa processor, microprocessor, controller, or a combination of processors,microprocessor, and/or controllers that performs instructions stored inthe volatile 215 or non-volatile memory 210 to manipulate data stored inthe memory. The non-volatile memory 210 can store the processorinstructions utilized to configure the playback device 200 to performprocesses including processes in accordance with embodiments of theinvention and/or data for the processes being utilized. In otherembodiments, the playback device software and/or firmware can be storedin any of a variety of non-transitory computer readable mediaappropriate to a specific application.

Servers

Some processes for providing methods and systems in accordance withembodiments of this invention are executed by the HTTP server; sourceencoding server; and/or local and network time servers. The relevantcomponents in a server that perform the processes in accordance withembodiments of the invention are shown in FIG. 3. One skilled in the artwill recognize that a server may include other components that areomitted for brevity without departing from the embodiments of thisinvention. The server 300 includes a processor 305, a non-volatilememory 310, and a volatile memory 315. The processor 305 is a processor,microprocessor, controller, or a combination of processors,microprocessor, and/or controllers that performs instructions stored inthe volatile 315 or non-volatile memory 310 to manipulate data stored inthe memory. The non-volatile memory 310 can store the processorinstructions utilized to configure the server 300 to perform processesincluding processes in accordance with embodiments of the inventionand/or data for the processes being utilized. In other embodiments, theserver software and/or firmware can be stored in any of a variety ofnon-transitory computer readable media appropriate to a specificapplication. Although a specific server is illustrated in FIG. 3, any ofa variety of server configured to perform any number of processes can beutilized in accordance with embodiments of the invention.

Playback Synchronization

In accordance with embodiments of this invention, playback devicesobtain time information from a common source and use the timeinformation to set the playback clock of the devices. The playbackdevices also receive playback time information from the stream contentprovider. The playback time information is then used to adjust thepresentation time of frames in the stream based upon the playback clock.Since the playback clocks have been set based upon time information froma common source there should by less than 10 s of ms delay between thepresentations. A timing diagram of messages and information passedbetween devices over a network in accordance some embodiments of thisinvention is illustrated in FIG. 4.

In FIG. 4, the devices include a first playback device, a secondplayback device, a media server, and a network time server. The firstand second playback devices are devices capable of presenting streamedencoded media content. The media server is a system that is providingthe streamed content over the network to the first and second playbackdevices. The network time server is a time server that provides timeinformation. In accordance with some embodiments, the time informationis provided using NTP. Furthermore, the time information provides timeinformation based upon Coordinated Universal Time (UTC).

In accordance with the timing diagram of FIG. 4, the first playbackdevice transmits a request 405 to the time server for time informationand receives time information 410 from the time server in response tothe request. Likewise, the second playback device transmits a request415 to the time server for time information and receives timeinformation 420 from the time server in response to the request.Furthermore, the media server also transmits a time information request425 to the time server and receives time information 430 from the timeserver in response to the request. In accordance with some embodiments,these exchanges are performed using NTP. However, other means forperforming these exchanges of messages may be used without departingfrom embodiments of this invention. Furthermore, one skilled in the artwill recognize that devices do not need to receive the time informationfrom the same time server as long as the time information provided bythe different time servers is generated from a common source. Oneskilled in the art will recognize that due to the network bandwidth andconfiguration of the networks connected to the devices that some delaymay be introduced between the time information provided to the variousdevices by the time server.

The first and second playback devices use the time information receivedto set the respective playback clocks in each playback device. The firstplayback device transmits a request 435 for OTT content or otherpresentation time sensitive content to the media server. The mediaserver transmits stream initiation information for a stream of encodedmedia for the desired content 440 to the first playback device.Similarly, the second device transmits a request 445 for the same OTTcontent or other presentation for time sensitive content to the mediaserver. The media server transmits stream initiation information for astream of encoded media for the desired content 450 to the secondplayback device. In accordance with some embodiments, the streaminitiation information includes a start time based on the UTC.

The media server then commences transmitting streams of the mediacontent 455 and 460 to each of the first and second playback devices.Each of the first and second devices then adjusts the Presentation TimeStamp (PTS) of each frame received in the stream of media content basedupon stream initiation information. In accordance with some of theseembodiments, the start time in the stream will be based upon theplayback clock derived from UTC and the UTC of the start time. Since thestart time of the stream is derived from the UTC and the playback clockis derived from the UTC, the delay between the presentations will be inthe 10 s of ms based on the difference of time information received bythe various devices.

In some embodiments, it may be desirable to have the synchronization ofplayback of the content include less than 10 s of ms of delay. This maybe the case where devices are providing playback of the content in closeproximity to one another. A timing diagram of a system that provides forgreater synchronization between playback devices in accordance with someembodiments of this invention is shown in FIG. 5.

The system in FIG. 5 includes a first playback device, a second playbackdevice, a local time server, a media server, and a network time server.The first and second playback devices are devices capable of presentingstreamed encoded media content and each of these devices is connected toa LAN. The LAN includes a local time server. The local time server is aserver that provides time information to devices connected to the LAN.In accordance with these embodiments, the time information is based upontime information received from a network time server as explainedfurther below. The media server is a system that is providing thestreamed content over the network to the first and second playbackdevices. The network time server is a time server that provides timeinformation. In accordance with some embodiments, the time informationis provided using NTP. Furthermore, the time information provides timeinformation specified in terms of UTC.

In accordance with the timing diagram of FIG. 4, the local time servertransmits a request 505 for time information to the network time serverand receives the time information 510 from the time server in response.The media server also transmits a time information request 515 to thetime server and receives time information 520 from the time server inresponse to the request. In accordance with some embodiments, theseexchanges are performed using NTP. However, other means for performingthese exchanges of messages may be used without departing fromembodiments of this invention. Furthermore, one skilled in the art willrecognize that servers do not need to receive the time information fromthe same time server as long as the time information provided by thedifferent time servers is generated from a common source. One skilled inthe art will also recognize that due to the network bandwidth andconfiguration of the networks connected to the servers that some delaymay be introduced between the time information provided to the variousdevices by the time server.

The local time server then uses the time information to generate timeinformation for use by the playback devices connected to the LAN. Thefirst playback device transmits a request 525 to the local time serverfor time information and receives time information 530 from the localtime server in response to the request. Likewise, the second playbackdevice transmits a request 535 to the local time server for informationand receives time information 540 from the local time server in responseto the request. The first and second playback devices use the timeinformation received to set the respective playback clocks in eachdevice. As the time information is received over a LAN from the localserver, there is less delay introduced into the time information. Thus,the playback clocks of the first and second playback devices will bemore closely synchronized.

To start playback, the first device transmits a request 545 for OTTcontent or other presentation time sensitive content to the mediaserver. The media server transmits stream initiation information for astream of encoded media for the desired content 550 to the firstplayback device. Similarly, the second device transmits a request 555for the same OTT content or other presentation for time sensitivecontent to the media server. The media server transmits streaminitiation information for a stream of encoded media for the desiredcontent 560 to the second playback device. In accordance with someembodiments, the stream initiation information includes a start timebased on the UTC.

The media server then commences transmitting streams of the mediacontent 565 and 570 to each of the first and second playback devices.Each of the first and second devices then adjusts the Presentation TimeStamp (PTS) of each frame received in the stream of media content basedupon stream initiation information. In accordance with some of theseembodiments, the start time in the stream will be based upon theplayback clock derived from UTC information received from the local timeserver and the UTC of the start time. Since the start time of the streamis derived from the UTC and the playback clock is derived from the UTCreceived from the local time server instead of the network time server,the delay between the presentations will typically be less than the 10 sof ms of the system described above in reference to FIG. 4. This isbecause the internal clocks of the individual playback devices are moreclosely synchronized due to the lack of latency between the individualplayback devices and the local time server of the LAN.

A process performed by a playback device to provide synchronizedplayback in accordance with some embodiments of this invention isillustrated in FIG. 6. The process 600 includes transmitting a requestfor time information to a time provider (605). As discussed withreference to FIGS. 4 and 5, the time provider may be, but is not limitedto, a network time server and/or a local time server. The devicereceives time information from the time provider (610). The timeinformation is preferably based upon the UTC. The time information isused by the playback device to set a media playback clock (615).

When the playback device plays back OTT or other presentation timecontent, the playback device receives stream initiation information(620). In accordance with some embodiments, the stream initiationinformation may be in the top level index of the requested content orprovided along with some other type of administrative information suchas metadata or the like. In accordance with some embodiments, the streaminitiation information includes a start time based on the UTC. Theplayback device then receives the stream of media content. As the streamis received, the playback device adjusts the presentation timeinformation of each frame based upon the stream initiation informationand the playback clock (625). The presentation time information may bethe Presentation Time Stamp (PTS) of a frame, PTS of a reference framefor a frame, or any other information that may be used to determine thetime at which a frame may be presented. In accordance with some of theseembodiments, the start time and the time of the playback clock are usedto adjust the PTS of each frame. In particular, the difference betweenthe start time received from the media server and the playback clock maybe used to adjust the PTS of each frame. The playback of the stream thencommences (630).

A process for providing stream initiation time information performed bya media server in accordance with embodiments of this invention isillustrated in FIG. 7. Process 700 includes transmitting a request fortime information to a network time provider (705). As discussed withreference to FIGS. 4 and 5, the time provider may be, but is not limitedto, a network time server. The media receives time information from thetime provider (710). The time information is preferably based upon theUTC. The time information is used by the media server to determine astream initiation time (715). As discussed above, the stream initiationtime may be a start time that is based upon the UTC.

The stream initiation time is then used to generate stream initiationinformation for the stream of content (720). When a request for a streamof the content is received (725), the stream initiation information isprovided to the requesting playback device (730) and the content isstreamed to the playback device (735).

The above is description of embodiments of systems and methods inaccordance with the present invention. It is foreseen that other skilledin the art will design alternative systems that infringe on thisinvention as set forth in the following claims either literally orthrough the Doctrine of Equivalents.

What is claimed is:
 1. A method for playback of encoded media performedby a playback device comprising: transmitting a request for timeinformation from a first playback device in a local network to a networktime provider for the local network; receiving the time information fromthe network time provider in the first playback device; transmitting arequest for the time information from at least a second playback devicein a local network to a network time provider for the local network;receiving the time information from the network time provider in atleast the second playback device of the plurality of playback devices;and setting a playback clock in each playback device that received thetime information using the received time information such that a timedelay among each set playback clock is less than tens of milliseconds.2. The method of claim 1 wherein the network time provider is a NetworkTime Protocol Server and the transmitting of the request and receivingof the time information are performed using Network Time Protocol. 3.The method of claim 1 wherein the time information is provided inCoordinated Universal Time.
 4. The method of claim 1 wherein the networktime provider is a local time server.
 5. The method of claim 4 furthercomprising: transmitting a request for time information from the localtime server to a Network Time Protocol server; receiving the timeinformation from the Network Time Protocol server in the local timeserver; receiving the request for time information from the playbackdevice; generating the time information for the playback device from thetiming information received from the Network Time Protocol server;transmitting the time information to each playback device that receivedthe time information.
 6. The method of claim 1 further comprising:receiving stream initiation information for a stream of encoded mediafrom a stream media provider in each playback device that received thetime information, wherein the stream initiation information includes astream start time; receiving the stream of encoded media in eachplayback device that received the time information, wherein the streamincludes a plurality of frames of the encoded content; adjusting thepresentation time information of each of the plurality of frames of thestream in each playback device that received the time information basedupon the stream initiation information and the time information, whereineach playback device utilizes the time information from its playbackclock; and presenting each of the plurality of frames using eachplayback device that received the time information based upon theadjusted presentation time information in each playback device havingreceived time information such that there is no noticeable delay of thepresentation of each of the plurality of frames among the playbackdevices receiving the time information.
 7. The method of claim 6 whereinthe start time is provided in Coordinated Universal Time.
 8. The methodof claim 6 further comprising: transmitting a request for timeinformation from the stream media provider to a Network Time Protocolserver; receiving the time information from the Network Time Protocolserver in the local time server; generating the stream initiationinformation for the stream of encoded media using the time informationreceived by the stream media provider wherein the stream initiationinformation includes the stream start time; receiving a request for theencoded playback from playback device that received the time informationby the stream media provider; transmitting the stream initiationinformation includes the stream start time from the stream mediaprovider to the playback device; and transmitting the stream of encodedmedia from the stream media provider to each playback device thatreceived the time information.
 9. A system of playback devices forproviding devices with synchronous playback of encoded media based on aprovided start time comprising: a local network; a network time providerin the local network; and at least a first and a second play back deviceconnected to the local network; wherein the at least first and secondplayback devices each comprises: memory; and a processor configured viaa client application stored in the memory to: transmit a request fortime information to the network time provider, receive the timeinformation from the network time provider, wherein the time informationis provided to at least the first and second play devices connected tothe local network, set a playback clock using the received timeinformation such that a time delay among each playback clock thatreceived the time information is less than tens of milliseconds.
 10. Thesystem of claim 9 wherein the network time provider is a Network TimeProtocol Server and the transmitting of the request and receiving of thetime information are performed using Network Time Protocol.
 11. Thesystem of claim 9 wherein the time information is provided inCoordinated Universal Time.
 12. The system of claim 9 wherein thenetwork time provider is a local time server.
 13. The system of claim 12wherein the time information provided by the local time server is basedon time information from a Network Time Protocol Server.
 14. The systemof claim 9, the processor is further configured via a client applicationstored in the memory to: receive stream initiation information for astream of encoded media from a stream media provider wherein the streaminitiation information includes a stream start time, receive the streamof encoded media wherein the stream includes a plurality of frames ofthe encoded content, adjust presentation time information of each of theplurality of frames of the stream in the playback device based upon thestream initiation information and the time information from its playbackclock, and present each of the plurality of frames using the adjustedpresentation time information such that there is no noticeable delay ofthe presentation of each of the plurality of frames among the playbackdevices receiving the time information.
 15. The system of claim 14wherein the start time is provided in Coordinated Universal Time.